Pulse-echo ultrasonic imaging technology has become a vital tool for clinicians for examining the internal structure of living organisms. In the diagnosis of various medical conditions, it is often useful to examine soft tissues within the body to show structural details of organs and blood flow in these organs. Experienced clinicians can use this information in diagnosing various pathologies.
To examine internal body structures, ultrasonic images are formed by producing very short pulses of ultrasound using a transducer, sending the pulses through the body, and measuring the properties of the echoes (e.g., amplitude and phase) from targets at varying depths within the body. Typically, the ultrasound beam is focused at various depths within the body in order to improve resolution or image quality. The echoes are received by a transducer, typically the same transducer used for transmission, and processed to generate an image of the object, usually referred to as a B-scan image.
Generally, the transmitted ultrasound beam is focused from pulse to pulse. It is common to create an image focused at a specific and operator selectable depth by focusing the ultrasound beam at that depth for all pulses. This is called single zone focusing. An extension of single zone focusing uses multiple sets of focused transmitted beams to create a composite image. Each set is focused at a specific depth. This is usually called multiple zone focusing. Generally, the number of zones varies from three to five zones. In this manner, real-time images are obtained with improved focus in certain regions in the image. This increase in image quality usually results in lower frame rates. Lower frame rates mean that moving structures or blood flow are not easily imaged and diagnosis may be impaired.
An ultrasound image is constructed by acquiring data along specific vectors or lines. In other words, a transmit pulse is sent from the transducer in a given direction and echos from scatters in that direction are received by the transducer. This process is commonly referred to as scanning the image. In multi-zone focusing, the data is acquired along each line using multiple transmit pulses, where a transmit pulse is sent along the line for each zone. After all of pulses for a particular line have been transmitted, then scanning continues at another line.
The scanning of an object normally occurs from one side of the transducer to the other. In this manner, adjacent vectors are fixed in order beginning at one side of the transducer and finishing at the other side. Because each line is comprised of multiple zones, each having its own transmit pulses, the transmission of the transmit pulses for the single line must be controlled to ensure that the reception of the echos from each of the pulses do not interfere with each other. This interference is referred to as axial interference. Axial interference corrupts the image.
One prior art solution for reducing axial interference is to include a guard band (i.e., a time period) between successively fired transmit pulses on the same line to allow the echos from previous transmit pulses to dissipate sufficiently before sending a subsequent transmit pulse. In this manner, the echos from the preceding pulse on the same line do not cause enough interference to degrade the image, thereby resulting in a higher quality image. It is desirable to reduce the axial interference due to the transmit vectors on the same line.
Another problem that may occur in scanning an image is the interference caused by adjacent vectors. This interference is referred to as lateral interference and is caused by the transmit pulse partially overlapping each other due to the expansion of the ultrasound signal within the object. The lateral interference causes the lateral resolution to be corrupted. It is desirable to reduce lateral interferences produced by adjacent transmit vectors.
Also during scanning of an object, signal ring-down may occur in which an ultrasound wave is distorted for an extended period of time due to layers of the object having different attenuations. The distortion can interfere with any ultrasound waves fired nearby. Signal ring-down occurs most often when imaging a fluid region, such as bladders and obstetrics, where the fluid region has such a different refractory index that the ultrasound wave becomes distorted. Therefore, it is also desirable to reduce signal ring-down when performing ultrasound imaging.
The present invention provide for altering the spacing between adjacent vectors, the timing of each vector, and the number of vectors needed to make an ultrasound image. In this manner, the present invention reduces axial interference, lateral interference and signal ring-down.